1. Field of the Invention
The present invention relates to a diaphragm pump, and more particularly, to a double-diaphragm pump which is suitably used to transport, transfer and recirculate a fluid such as a liquid, powder or glanular material.
2. Description of the Prior Art
In a typical conventional pressure chamber (pressure vessel or outer chamber) of a diaphragm pump, a domed outer chamber which has generally a uniform thickness is produced using a material such as aluminum, cast iron, engineering plastic, stainless steel, etc., and such an outer chamber is rigidly secured to the body of the pump to thereby define a material chamber for pumping a material such as a fluid or the like. The wall thickness of the shell portion of the outer chamber is substantially uniform throughout it as described above, and in order to enable the pressure chamber that serves as a pressure vessel to bear high pressure, it is conventional practice to uniformly increase the wall thickness of the shell portion to thereby enhance the pressure resistance. The shell portion thus formed may be deformed considerably by the constantly high pressure within the material chamber, or local stress concentration may be caused in the shell portion by rapid increase and decrease in pressure, and there is therefore a fear of the shell portion developing a small but dangerous crack. In order to overcome this problem, it has been attempted to reinforce the shell portion by providing radial ribs on its outer surface. However, in this prior art, the outer chamber and the diaphragm are rigidly secured to the body of the pump by fastening them together in one unit at the outer edge of the shell portion by means of bolts which are received through bores provided in the shell outer edge. Accordingly, that portion of the outer edge of the shell portion which is defined between each pair of adjacent fastened portions (i.e., the portion intermediate between each pair of adjacent bolt receiving bores) may be deflected (expanded outward) by high pressure, and this non-uniform deformation may cause leakage of a fluid from the outer edge of the shell portion.
In order to enhance the pressure resistance and prevent the local deformation, it is conventional practice to use an excessively large amount of a material for forming the outer chamber and considerably increase the weight of the pressure vessel or the overall weight of the pump. However, this practice goes against the tendency to reduce the amount of material used and the weight of the product. When the outer chamber is formed from an engineering plastic, the chamber may be formed with a more than enough wall thickness, but, since the heat capacity increases in proportion to the weight of the plastic used, if the chamber has a large wall thickness, a long time is required for the formed material to cool down. In addition, non-uniform cooling takes place in the shell portion, and this leads to small strains or deflections on the product, which may result in lowering in the pressure resistance and leakage resistance of the chamber. More specifically, when the outer chamber is produced from a thermoplastic material by a molding process, it has been demanded to minimize the amount of material used to thereby enable the formed material to cool down relatively quickly and to provide a structure which enables the applied pressure and the cooling rate to be made as uniform as possible throughout the shell portion and which imparts high pressure resistance to the product.